In recent years, many applications have been found for the integration of organic light emitting diodes (OLED) with standard CMOS circuits, for example in near-to-eyes lenses. The integration of OLED and CMOS into the same chip can provide many benefits, such as compact size, fast speed, low power consumption etc.
In a typical OLED on MOS structure for use as a display, a wafer (e.g. a silicon wafer) having an integrated circuit is provided. Metal pixels are photolithographically patterned on the top of the wafer. An organic compound layer is then formed on top, covering the metal pixels as well as the gaps between the metal pixels.
However, the present inventors have appreciated that problems may arise since the standard CMOS process is not well suited to the integration of OLED on CMOS, for at least two reasons. Firstly, the typical metal deposition techniques often do not result in a smooth surface with a surface roughness of less than 3 nm, for the application of OLED electrodes. The roughness of the surface of the OLED bottom electrodes (i.e. of the top surface of the metal pixel) is believed to cause the lifetime of the OLED to shorten. Secondly, the thickness of metals employed in CMOS processes is commonly more than 6000 Å (600 nm), with the result that the pixel gaps (or metal gaps) are deeper. OLED organic materials deposited in the metal gaps can induce side wall emitting and high electrical fields at the metal edge. As a result, both the efficiency and reliability of the OLED device is degraded.
To address these problems it is in principle possible to reduce the thickness of the metal used for forming the electrodes of the OLED, and the roughness could to be optimized by adjusting the metal deposition parameters such as power, temperature and deposition rate. Nonetheless, the existing solutions address the problems only in part, or introduce further problems. Firstly, there are limitations as to how much the thickness of the electrodes can be reduced. A very thin aluminium film does not have a good reflectivity over the entire visible light range. Secondly, an over etch is normally necessary to ensure that no metal bridges remain between adjacent pixels, and this also limits the reduction of the inter-metal gap depth owing to variations in the metal thickness. Thirdly, by reducing the thickness of the OLED electrodes, the OLED efficiency and reliability degradation can only be alleviated to some extent, but not completely prevented.